A radiographic phosphor panel contains a layer of phosphor, a crystalline material which responds to X-radiation on an image-wise basis. Like many other crystalline materials, radiographic phosphors have a crystal matrix which allows for the replacement of some atoms by other similar atoms, but does not readily accept other atoms or moieties. Radiographic phosphor panels can be classified, based upon their phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels. Intensifying panels are used to generate visible light upon exposure of the intensifying panel to X-radiation. A sheet of photographic film is positioned to intercept the visible light generated and commonly is pressed against the intensifying panel within a light-tight cassette. Other prompt emission panels operate similarly, but in place of the photographic film have some other means for visualizing the X-radiation.
Storage panels have storage phosphors, that have the capability of storing latent X-ray images for later release, apparently by locally trapping electron-hole pairs created by incident X-rays. Storage phosphors are distinguishable from the phosphors used in X-ray intensifying or conversion screens. In the latter, a latent image is not stored and X-radiation causes the immediate release of visible light from irradiated phosphor crystals.
Radiation image storage panels are used in computed radiography. The panel is first exposed to X-radiation to create a latent image. The panel is then stimulated with longer wavelength radiation, resulting in the emission of radiation at a third wavelength. Typically a laser having a red or infrared beam is scanned over the panel, resulting in the emission of green or blue radiation. The emitted light is collected and the resulting signal is processed electronically to produce a final image.
Alkaline earth metal fluorohalide storage phosphors are described in a number of patent publications. U.S. Pat. No. 4,926,047 teaches a radiation image storage panel including storage phosphors designated by the general formula: EQU MIIFX.aMIX'.bM'IIX".sub.2.cMIIX'".sub.3.xA:yEu.sup.2+ ;
in which MII is Ba, Sr, and/or Ca; MI is Li, Na, K, Rb, and/or Cs; M'II is Be and/or Mg; MIII is Al, Ga, In and/or Tl; A is metal oxide; X is Cl, Br, and/or I; X', X", and X'"are F, Cl, Br, and/or I; a is from 0 to 2; b is from 0 to 10.sup.-2 ; c is from 0 to 10.sup.&lt;2 ; a+b+c is greater than or equal to 10.sup.-6 ; x is from 0 to 0.5; and y is from 0 to 0.2. This patent further teaches that metal oxides may be included in the starting materials for MFX type storage phosphors. Although it is not completely understood how the oxide improves the performance of the phosphor, advantages such as increased luminescence, improved afterglow, and sinter prevention have been mentioned. It is also not known how metal oxide is incorporated into phosphor. There are a number of possibilities, for example, incorporation within the host lattice, and adherence to the surface of crystals.
It has generally been taught in the art that the optimal MFX-type storage phosphor is BaFBr activated with 0.001 mole percent Eu.sup.2+ and containing, as addenda, sodium halide, preferably sodium bromide.
U.S. Pat. No. 5,227,254 to Brixner et al. states:
"BaFBr:Eu is also the preferred phosphor for use in photostimulable storage panels." PA1 "From the viewpoint of enhancement of the luminance of stimulated emission of the phosphor, each of X', X" and X'" in the formula (I) [see above quotation from U.S. Pat. No. 4,926,047] are preferably Br or I, and particularly preferred is Br. MI is preferably Li or Na, and particularly preferred is Na." PA1 "The phosphor used in the radiation image recording and reproducing method is improved in the luminance of stimulated emission by introducing a suitable amount of sodium halide (NaX') into a divalent europium activated barium fluorohalide phosphor (BaFX:Eu.sup.2+) which is one of divalent europium activated alkaline earth metal fluorohalide phosphors. PA1 Further, the above mentioned effect of the added NaX' to enhance the luminance of stimulated emission of the resulting phosphor has been confirmed in a divalent europium activated strontium fluorohalide phosphor (SrFX:Eu.sup.2+) as well as in a divalent europium activated calcium flurohalide phosphor (CaFX:Eu.sup.2+). In other words, it has been confirmed that a divalent europium activated alkaline earth metal fluorohalide phosphor containing a sodium halide and having the following formula, exhibits the stimulated emission of higher luminance than a divalent europium activated alkaline earth metal fluorohalide phosphor containing no sodium halide: EQU M.sup.II FX.xNaX':yEu.sup.2+ PA1 in which M" is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Ca; each of X and X' is at least one halogen selected from the group consisting of Cl, Br, and I; and x and y are numbers satisfying the conditions of 0&lt;x&lt;=2, 0&lt;y&lt;=0.2, respectively". PA1 "the phosphor containing three kinds of elements of fluorine, bromine and iodine as halogen which is a host component of the phosphor is prominently enhanced in the luminance of stimulated emission. The radiation image recording and reproducing method employing said stimulable phosphor can be remarkably enhanced in the sensitivity." PA1 "[A]ctivated iodide phosphors are extremely hydroscopic. Absorption of small amounts of water rapidly reduces the conversion efficiency to a vanishingly small value. In order to employ activated iodide phosphors it is therefore necessary to provide the activated iodide phosphor in the screen in a form in which it remains stable for long periods of time. PA1 "Various ways have been taught for using thallium activated potassium iodide and protecting the iodide from moisture." (Bates et al, column 1, lines 20-30)
European Patent Application No. 0 107 192 A1 by Takahashi et al describes a phosphor like that of U.S. Pat. No. 4,926,047. This application states:
U.S. Pat. No. 4,505,989 to Umemoto et al teaches a radiation image storage panel including storage phosphors designated by the general formula: EQU M"FX.xNaX':yEu.sup.2+ :zA;
in which M" is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Ca; each of X and X' is at least one halogen selected from the group consisting of C1, Br, and I; A is at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co and Ni; and x, y, and z are numbers satisfying the conditions of 0&lt;x&lt;=2, 0&lt;y&lt;=0.2 and 0&lt;z&lt;=10.sup.-2. The Umemoto patent states:
Divalent europium activated alkaline earth metal fluorohalide containing iodide is not distinguished from other phosphors within the scope of the Umemoto patent.
European Patent Application No. 0 142 734 A1 teaches a phosphor described by the formula: EQU BaF(Br.sub.1-x I.sub.x):yEu.sup.2+
This application states that:
There is no teaching or suggestion as to use of addenda.
Degradation of final images due to panel discoloration has long been recognized for intensifying screens. Radiation image storage panels, unlike intensifying screens, are subject to degradative losses of both emitted light and stimulating radiation. Since these effects are cumulative, discoloration can be an even more serious issue in storage panels than in intensifying screens.
Yellowing of a phosphor layer of a radiation image storage phosphor panel, in which the phosphor contains iodine, is described in European Patent Specification No. EP 0 234 385 B1. The yellowing is ascribed to liberation of free iodine. The phosphor is divalent europium activated alkaline earth metal fluorohalide phosphor containing iodine or bismuth activated alkali metal halide phosphor containing iodine. The solution described for the yellowing problem, is incorporation in the phosphor layer of a compound containing a free epoxy group and/or a compound selected from: phosphites, organotin compounds, and specific metal salts of organic acids.
U.S. Pat. No. 4,374,905, to Rabatin, teaches a solution to both discoloration of an intensifying screen by "volatile organic constituents" and attack by water. The phosphor for an intensifying screen was milled with anhydrous MgSO4 or ZnSO4 during preparation of the screen. It was proposed that the protective action was based upon the reaction: EQU MgSO.sub.4 +2HOH.fwdarw.Mg(OH).sub.2 +2H.sup.+ +SO.sub.4.sup.2-
U.S. Pat. No. 3,836,784, to Bates et al, teaches that small amounts of "stabilizers", such as sodium thiosulfate or potassium thiosulfate can be included in the fluorocarbon binder of an intensifying screen. Bates et al, which used an iodide containing phosphor, noted:
U.S. Pat. No. 3,023,313 to De La Mayer et al teaches the addition of small amounts of sodium thiosulfate or potassium thiosulfate to the polymer binder of an intensifying screen.
In U.S. Pat. No. 4,360,571, to Rabatin, phosphors were treated with fatty acids or metal salts of fatty acids to prevent discoloration by "volatile organic constituents" and attack by water.
In GB 2 017 140 A, intensifying screens were stabilized against discolouration and hydrolysis by incorporation of a compound containing a free epoxy group and, optionally, a dialkyl tin compound such as dibutyl tin dioctyl as an additional stabilizer.
As a matter of convenience, a material which can be added to phosphor panels or compositions for the purpose of stabilizing against iodine related yellowing is referred to herein as a "stabilizer for iodine" or "iodine stabilizer".
It would be highly desirable to provide a storage phosphor and image storage panel composition which is markedly improved in its sensitivity compared to those discussed above.